Stent matrix

ABSTRACT

An object of this invention is to provide a radially expandable stent ( 1 ) that holds a passageway enlarged by placing the stent ( 1 ) into a lumen. The stent ( 1 ) comprises a cylindrical frame formed by a plurality of unit structures ( 11, 12, . . . 16 , . . . ); said unit structures ( 11, 12, . . . 16 , . . . ) formed into a closed zig-zag configuration including an endless series of straight sections ( 111 ) and joined by bends ( 112 ), and arranged face to face into a shape of multistage; connecting members ( 31, 33, 35, 37 . . .  ), which connect said unit structures ( 11, 12, . . . 16 , . . . ); and a mesh ( 91 ), which is wrapped around an outside of said frame.

FIELD OF THE INVENTION

[0001] This invention relates to an elongate stent matrix which definesa surface in a closed loop surrounding an elongate flow path, and whichis capable of expansion during deployment in a bodily lumen, from asmall diameter delivery configuration to a large diameter lumenwall-supporting configuration;

[0002] the matrix exhibiting a multiplicity of cells formed from struts,each of which cells has, at least in the delivery configuration, alength dimension along said flow path, a width dimension within saidclosed loop perpendicular to said flow path, and a thicknessperpendicular to the length and width of the cell, with a first band ofsaid cells at or near a first end of said matrix, and a further band ofsaid cells at or near a second end of said matrix, opposite the firstend;

[0003] the matrix further exhibiting a first ring which includes atleast one free vertex.

[0004] Thus, this invention relates to prostheses to maintain patency ofbodily lumens, and to precursors for such prostheses. The term “stent”is adopted, to signify such prostheses, because it is well-known andunderstood by those skilled in the art. Readers should appreciate thatthe term “stent” in this specification is to be understood to embraceall those prostheses which are useful for maintaining patency of abodily lumen, whether, or not they are conventionally referred to, bythose skilled in the art, as examples of stents.

[0005] It will also be understood that, in some applications, a stentmatrix as described above is useful in itself whereas, in otherapplications, it requires some form of covering of a portion of itssurface area, in order to be effective. Such a covered embodiment issometimes referred to as a “stent graft” or as a “covered stent”. Thescope of the present invention includes such embodiments.

[0006] It is by now well-known that there are two prominent categoriesof stent, namely, self-expanding stents and stents which are expanded byinflation of a balloon within the flow passage of the stent, to causeplastic deformation of the metallic material making up the stent matrix,as it moves from its small diameter delivery configuration to its largediameter deployed configuration. Self-expanding stents are often made ofa shape memory material, which normally is the nickel-titanium alloyknown as NITINOL.

[0007] The art of covering metallic stent matrixes is alsowell-developed. For example, the present Applicant has much experiencein covering stent matrixes with expanded polytetrafluoroethylenematerial and there is an extensive body of patent literature disclosingsuch technology.

[0008] It is also well-known that stents are designed and built withparticular applications in mind. Stents for maintaining the esophaguspatent are of a different order of size from those constructed formaintaining open a coronary arterial lumen. The present invention isbelieved to be applicable to a wide range of stent applications, but onefound particularly useful now is in the field of stents for theesophagus.

[0009] In designing stents for the esophagus, there are a number oftensions between opposing design objectives. First, there is a tensionbetween a need to maintain a lumen diameter big enough to accommodatesolid food as it is swallowed, and the need to avoid excessive traumaand pressure on the bodily tissue walls of the esophagus. Second, thereis a tension between the need to anchor the stent securely in theesophagus so that the stent will not migrate along the length of theesophagus, and the need to avoid excessive trauma of the bodily tissueof the walls of the esophagus. Third, adequate radiopacity of the stentis needed, for tracking its location, but provision of sufficient bulkof dense material to achieve this objective tends to conflict with theoverriding objective of achieving sufficient patency. To a greater orlesser extent these tensions can also be found in other stentapplications, to which this invention also applies.

[0010] It hardly needs to be noted that stent designs are feasible onlyif the necessary manufacturing technique to realize the design are notexcessively difficult, and not excessively expensive. The stent matrix,once placed in the body, must be inert in the bodily environment, andnon-toxic. As the matrix is immersed in an electrolyte, the design mustdeny opportunities for deleterious electrochemical activity to occur,such as dissolution, corrosion and galvanic activity.

[0011] The present invention aims to provide improvements in the designcompromises indicated immediately above.

BACKGROUND PRIOR ART

[0012] For the disclosure of an esophageal stent see WO 92/06734. For adisclosure of stents made of Nitinol, see WO 94/17754. For a disclosureof stents covered in expanded PTFE, see for example U.S. Pat. No.5,749,880. WO 97/16133 discloses a stent fabricated by braiding offilaments. At each end of the stent is a ring of beads, created byfusing the material of the filaments of two intersecting stents of thebraid. All of these documents are incorporated by reference in thisspecification.

[0013] Stents and similar endoluminal devices are currently used bymedical practitioners to treat tubular body vessels or ducts that becomeso narrowed (stenosed) that flow of blood or other biological fluids isrestricted. Such narrowing (stenosis) occurs, for example, as a resultof the disease process known as arteriosclerosis. While stents are mostoften used to “prop open” blood vessels, they can also be used toreinforce collapsed or narrowed tubular structures in the respiratorysystem, the reproductive system, bile or liver ducts or any othertubular body structure including the esophagus. However, stents aregenerally mesh-like so that endothelial and other tissues can growthrough the openings resulting in restenosis of the vessel.

[0014] Polytetrafluoroethylene (PTFE) has proven unusually advantageousas a material from which to fabricate blood vessel grafts or prostheses,tubular structures that can be used to replace damaged or diseasedvessels. This is particularly because PTFE is extremely biocompatiblecausing little or no immunogenic reaction when placed within the humanbody. This is also because in its preferred form, expanded PTFE (ePTFE),the material is light and porous and is readily colonised by livingcells so that it becomes a permanent part of the body. The process ofmaking ePTFE of vascular graft grade is well-known to one of ordinaryskill in the art. Suffice it to say that the critical step in thisprocess is the expansion of PTFE into ePTFE. This expansion represents acontrolled longitudinal stretching in which PTFE is stretched to severalhundred percent of its original length.

[0015] Apart from use of stents within the circulatory system, stentshave proven to be useful in dealing with various types of liver diseasein which the main bile duct becomes scarred or otherwise blocked byneoplastic growths, etc. Such blockage prevents or retards flow of bileinto the intestine and can result in serious liver damage. Because theliver is responsible for removing toxins from the blood stream, is theprimary site for the breakdown of circulating blood cells and is alsothe source of vital blood clotting factors, blockage of the bile ductcan lead to fatal complications. A popular type of stent for use in thebiliary duct has been formed from a shape memory alloy (e.g. Nitinol)partially because such stents can be reduced to a very low profile andremain flexible for insertion through the sharp bend of the bile ductwhile being self-expandable and capable of exerting a constant radialforce to the duct wall.

[0016] Cellular infiltration through stents can be prevented byenclosing the stents with ePTFE. Early attempts to produce a stentcovered by ePTFE focussed around use of adhesives or physical attachmentsuch as suturing. However, such methods are far from ideal and suturing,in particular, is very labour intensive. More recently methods have beendeveloped for encapsulating a stent between two tubular ePTFE memberswhereby the ePTFE of one member touches and bonds with the ePTFE of theother member through the mesh opening in the stent. However, such amonolithically encapsulated stent may tend to be rather inflexible.Therefore, there is a need for a stent covered to prevent cellularinfiltration and yet still flexible to ensure ease of insertion anddeployment and to accommodate extreme anatomical curves.

SUMMARY OF THE INVENTION

[0017] A combination of technical features which defines the presentinvention is set out in independent claim 1 below. According to thepresent invention, a stent matrix is characterized by the fixing of animported bead to the matrix, this bead having a thickness greater thanthe thickness which characterizes the struts of the matrix The beaddefines a female receiving portion, and the matrix includes a maleextending portion which co-operates with the receiving formation. Thegreater thickness of the bead is useful for X-ray visualisation and forsecure attachment of the bead to the matrix. By “imported” is meant thatthe bead does not originate from within the matrix, but is brought infrom a source other than the matrix itself.

[0018] For a stent matrix made of Nitinol, the struts of the matrixtypically have a thickness up to about 0.3 mm and a width of within arange of from about 0.16 to about 0.20 mm. In the experimental work onesophageal stents so far conducted by Applicant, it has been found thata bead diameter in a range of from about 0.9 to about 1.1 mm iseffective.

[0019] One basic structure of a self-expanding Nitinol stent is acylinder of Nitinol material characterized by a multiplicity of shortslits, in the longitudinal direction of the stent cylinder, these slitsbeing arranged in successive rings of slits along the lengths of thestent, each ring being staggered circumferentially from the nextadjacent rings, by regular intervals along the length of the stent. Theslits of every second ring are co-linear. Then, when such a stentcylinder is expanded radially, a pattern of diamond-shaped openingsappears. The length of the slits, and the circumferential spacings, areorganized so that each diamond has a length direction parallel to thelength of the stent cylinder, and a width direction around thecircumference of the cylinder, when the stent is in its fully extendedconfiguration.

[0020] In one useful embodiment of the present invention, each freevertex of each diamond-shaped cell at both ends of the stent cylinderhas an axially-extending cantilever strut which serves as a male portionon which may be fitted one of the imported beads.

[0021] In another preferred embodiment of the present invention, everyother such cantilever strut, at each end of the stent, is fitted with animported bead.

[0022] In a specially preferred embodiment of the invention, stentmatrixes which will receive a ring of beads at each end are modifiedfrom a strictly cylindrical configuration, in that the ring of cells ateach end of the stent is worked upon, so as to incorporate a degree ofoutward flaring, relative to the generalized envelope of the stentbetween its two ends. The outward flaring tends to enhance the anchoringpower of the stent ends in the bodily tissue of the lumen in which thestent has been installed, but the provision of the imported beads on thefree vertices of each flared end will tend to ameliorate the degree oftrauma to which the flared ends might otherwise subject the tissue intowhich they protrude.

[0023] In one preferred configuration for installation in the esophagus,the stent features ends which are outwardly flared by 15° relative tocylindrical end zones of the stent. Further, the esophageal stentfeatures a mid-length zone which is cylindrical but of a diametersmaller than the flanking end zone cylinders, there being a steplesstransition of diameter connecting each cylindrical end zone to thecylindrical mid-length zone. Further, the mid-length zone, but not thecylindrical end zones, is covered with a graft material, preferablyexpanded PTFE.

[0024] Nitinol stents can be formed from tubular material, or frominitially flat material which, after laser cutting of the aforementionedslits, is then formed into a tube. Otherwise, stent matrixes can beetched from sheet material, either tubular or flat. For example, astainless steel tube can be etched to make a stent which undergoesplastic deformation upon expansion by a balloon.

[0025] In a particularly preferred embodiment, the material of theimported beads is the same as that of the stent matrix, typically,stainless steel or Nitinol.

[0026] Alternatively, for the material of the imported beads, a materialcould be selected which has more or less the same electrochemicalpotential as that of the material of the stent matrix. For example, aNitinol stent could be fitted with beads of Tantalum, which has almostthe same electrochemical potential and greater radiopacity. Otherwise,each bead could be maintained electrically insulated from the matrix, asby an insulating layer for example a polymer.

[0027] In cases in which the bead and matrix material are the same theycan be fixed to each other by welding. Otherwise, each bead could befixed to the matrix by a mechanical engagement of co-operating surfaces,or by an intervening layer of adhesive, or by a tie layer of metalcompatible with both the stent and the bead. In one preferredembodiment, each bead is spherical and defines a radially-extendingrecess which receives the cantilever strut to which the bead is fixed.The recess can be a bore through the entire bead. The radiopacity of thezone of the stent in which the beads are located is thereby enhanced.Further enhancements in radiopacity may be achieved by coating the beadin highly radiopaque material such as gold or tantalum.

[0028] In many stent applications, it is important that the deliveryconfiguration of the stent exhibits as small a diameter as possible.Providing a relatively large diameter bead on every free vertex at eachend of the stent will tent to limit the degree of compression ofdiameter which can be achieved at the beaded ends of the stent. Thus, inone specially preferred embodiment of the present invention, when anespecially small diameter delivery configuration is needed, a bead isprovided on a strut of every other free vertex at each end of the stent,rather than on every vertex. However, for delivery to the esophagus, asomewhat larger diameter delivery configuration, relative to theinstalled diameter configuration, is acceptable, which leaves room toplace a bead on every one of the free vertexes at each end of theesophageal stent.

[0029] In applications where the objective of minimizing trauma dictatesthere should be a bead on every free vertex of the end ring of thestent, yet there is not enough room in the delivery configuration for somany spherical beads, it is contemplated to provide every free vertexaround the end ring with a non-spherical bead having more or less theshape of a convex-ended cylinder with its long axis aligned with itsfemale receiving portion.

[0030] In applications in which trauma is not a problem, beads can bepositioned at will, depending on where enhanced radiopacity is needed.Thus, beads could be provided at points of importance along the lengthof the stent, or around the circumference of the stent. In one example,one or more beads could be placed near a fenestration in the stent wall,to be put into registry with a side branch of the lumen in which thestent length is being installed.

[0031] For a better understanding of the invention, and to show how thesame may be put into effect, reference will now be made to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a perspective view of an esophageal stent graft;

[0033]FIG. 2 is a cross-sectional view along the line 2-2;

[0034]FIG. 3 is a cross-sectional view along the line 3-3;

[0035]FIG. 4 is an overview picture of the deployment of the graft;

[0036]FIG. 5 is a close-up view of the device being partially deployed;

[0037]FIG. 6 is a close-up view of the device fully deployed;

[0038]FIG. 7 is a picture of a fully encapsulated stent being tested forflexibility;

[0039]FIG. 8 is a picture of the FIG. 1 graft being tested forflexibility in the same manner as FIG. 7;

[0040]FIG. 9 shows an especially flexible stent design (the “Flexx”stent) preferred for use in the present invention; here the Flexx stentis shown in its expanded state;

[0041]FIG. 10 shows the flexible stent of FIG. 9 after it has been cutbut before it has been expanded;

[0042]FIG. 11 shows a close-up of the strut structure of the expandedstent of FIG. 9;

[0043]FIG. 12 shows a close-up view of the flexible stent design of FIG.9 immediately after being cut from a metal tube and before beingexpanded into the form of FIG. 11; and

[0044]FIG. 13 is a view of a fragment at one end of the FIG. 9 stent,showing the subject matter of the present invention;

[0045]FIG. 14 is a diametral section through one of the beads in FIG.13, in a plane tangential to the stent cylinder;

[0046]FIG. 15 is a diametral section through the FIG. 14 bead, in aplane transverse to a diametral plane of the stent, and the plane ofFIG. 14;

[0047]FIG. 16 is a section through the diameter of the stent, showing aleading end of the stent entering a loading machine;

[0048]FIG. 17 is a section as in FIG. 16, showing the stent leadingadvanced from the FIG. 16 position; and

[0049]FIG. 18 is a side view of a fragment of one end of the FIG. 1stent and (in section) of a bead to fit on that fragment, as well as thebead fitted to the fragment.

DETAILED DESCRIPTION

[0050] The present invention is useful for a covered stent device thatis virtually as flexible as an uncovered stent. Such flexibility isaccomplished by covering a stent on a first surface while limitedregions are covered on the opposite surface to ensure fixation of thefirst surface covering.

[0051] Referring now to the drawings, in which like reference numbersrepresent similar or identical structures throughout, FIG. 1 illustratesa preferred embodiment of stent graft which is particularly well-adaptedfor incorporation of the present invention. A partially encapsulatedstent-graft 10 is created by covering the abluminal surface of a stent12 with a biocompatible barrier material that is able to seal fistulaeand aneurysms and prevent or reduce tissue ingrowth from neointimalhyperplasia or tumour growth. In the preferred embodiment, the materialused for this purpose is a tubular layer of expandedpolytetrafluoroethylene (ePTFE) 20. the preferred ePTFE is one optimisedfor bond strength as described in U.S. Pat. No. 5,749,880. The stent 12in the preferred embodiment is a shape memory alloy stent havingenhanced flexibility, although stents of a variety of designs are usablewith the current invention. Also, the stent 12 can be made out of anytype of material besides shape memory alloy.

[0052] It is known to those of skill in the art that at a covering overat least one of the surfaces (luminal or abluminal) of a stent canprevent tissue ingrowth. Furthermore, the covering can be bonded to thestent to prevent it from coming detached and perhaps forming a blockagein the vessel. Although ePTFE has numerous favourable properties, it isrelatively difficult to attach it to a stent. Mechanical fasteners suchas sutures have the disadvantage of interrupting the integrity of theePTFE sheet so that leaking can occur. Although ePTFE does not adherewell to a stent, it can be made to bond to itself. Therefore, oneeffective method of affixing the ePTFE cover is to place ePTFE covers incontact with both the abluminal and luminal surfaces of the stent sothat one ePTFE covering can bond to the other where the ePTFE coveringstouch through opening in the stent. The drawback with this approach isthat the structural members of the stent are tightly surrounded and heldby ePTFE. When the stent bends or expands, the stent structural membersmust move relative to each other. This movement is resisted by thetightly adhering ePTFE (or other covering material).

[0053] Movement of the stent members relative to each other isfacilitated by limiting the region of the stent in which the structuralmembers are surrounded (encapsulated) by ePTFE. In a preferredembodiment the regions of encapsulation, which ensure attachment of thecovering to the stent, are limited to areas near the ends of the devicebut spaced from those ends. For a relatively short device theseend-encapsulated regions are more than adequate to afford attachment ofthe covering. If necessary one or more additional regions ofencapsulation could be added along the length of the device if it isfound necessary for stability of the covering. Clearly, the greaterpercentage of length of the device that is fully encapsulated, the morethe flexibility of the overall structure will be impeded. The ends areleft uncovered, and are flared outwardly. This helps to prevent unwantedaxial migration of the stent in the lumen. In other words, the flaredend helps to anchor the stent in the walls of the lumen.

[0054] An additional advantage of the limited encapsulation is thepossibility of enhanced healing. It is known that living cells willinfiltrate sufficiently porous ePTFE and the microcapillaries may formwithin and across the ePTFE wall so that a living intima is formed alongthe luminal surface. Where two layers of ePTFE surround the stent, itmay be significantly more difficult for cellular infiltration across thewall to occur. Although the figures show the continuous covering placedon the abluminal surface of the device, the illustrated embodiment alsolends itself to placement of the continuous covering on the luminalsurface. The configuration choice may depend on the precise applicationof the device. In some palliations, for example, large vessels having ahigh rate of blood flow, placing the covering on the luminal surface mayresult in advantageous lamellar flow of blood, that is to say, bloodflow without significant turbulence. There is some evidence that contactof the blood with a metal stent may result in local, limited thrombosis.While this may be detrimental, there is also some evidence that somelimited thrombosis results in enhanced healing. An advantage of using afull luminal covering could be improved anchoring of the device withinthe duct or vessel afforded by interactions between the bare abluminalstent and the duct or vessel wall. Therefore, the optimal configurationwill have to be empirically determined in many cases.

[0055] In the illustrated design (FIG. 1) the extremities 14 of thestent 12 are left completely uncovered and flare outward to facilitateanchoring of the stent within the vessel following expansion of thestent in situ. It will be apparent that this flared region is a featureof this particular embodiment and is not a required element of theinstant invention. The luminal surface of the stent 12 is covered atends 22 defined between points A and B and points C and D in FIG. 1, butis left uncovered in mid-section 24 defined between points B and C. Byleaving the mid-section 24 uncovered, the stent has increasedflexibility as well as reduced profile when compressed. The materialused to cover the ends 22 on the luminal surface of stent 12 isgenerally the same material that is used to cover the abluminal surface,and in FIG. 1 this material is ePTFE 30 (see FIG. 2), though any othersuitable biocompatible material could be used in the present invention.

[0056] Again, it is important to note that while the continuous tubularlayer of ePTFE 20 is shown on the abluminal surface of FIG. 1, is itpossible, and advantageous in some cases, to place a tubular layer ofePTFE on the luminal surface, while placing limited rings of ePTFE onlyon the abluminal surfaces at the ends of the device. Distances A-B andC-D in FIG. 1 can be lesser or greater, depending on the need forflexibility in the particular application. Moreover, there can be anynumber of encapsulated region(s) and these region(s) can be located indifferent areas of the stent. Also, while the preferred embodiments useencapsulated regions that extend completely around a circumference ofthe device (e.g. rings of material) as indicated by region 32 in FIG. 1,there is no reason that discontinuous regions of encapsulation cannot beused. Attaching discrete pieces of strips of ePTFE to a mandrel beforethe stent is placed on the mandrel can be used to form suchdiscontinuous regions. The size, shape and pattern formed by regions 32can be selected to enhance flexibility, etc. This allows differentregions of the device to exhibit different properties of flexibility,etc.

[0057] One the appropriate ePTFE covering is placed onto the luminal andabluminal surfaces, the ends 22 of the stent graft 10 are encapsulatedby connecting or bonding the luminal covering to the abluminal covering.Encapsulation can be accomplished by a number of methods includingsintering (e.g. heating), suturing, ultrasonically welding, stapling andadhesive bonding. In the preferred embodiment, the stent-graft 10 issubjected to heat and pressure to laminate (bond) the tubular ePTFElayer 20 on the abluminal surface to the two rings of ePTFE 30 (FIG. 2)on the luminal surface.

[0058]FIGS. 2 and 3 illustrate cross-sections of FIG. 1. A cross-sectionof stent-graft 10 is taken along line 2-2, through an end 22 of thedevice 10 in FIG. 2 and long line 3-3 through the mid-section 24 in FIG.3. These two cross-sections are shown to illustrate the additional layerof ePTFE 30 that is present on the luminal surface of the end 22 and notpresent on the luminal surface of the mid-section 24. As mentioned, thereason for encapsulating only the ends 22 of stent-graft 10 is toincrease its flexibility over a fully encapsulated stent, therebyallowing it to be bent into extreme curves without kinking. Most of thelength of the device is covered by only a single layer of ePTFE which isextremely flexible and which does not strongly interact with the stent.Therefore, the flexibility of the single layer area is essentially thatof the underlying stent device. FIG. 7 shows a fully encapsulated shapememory alloy stent bent in essentially as sharp a curve as possible.Note that the covering material is showing kinks or distortions due tothe inability of the covering material to move longitudinally relativeto the stent structural members. FIG. 8 shows an identical shape memoryalloy stent covered according to the present teaching. Only zonestowards each end of the device are fully encapsulated. Note that thedevice is capable of being bent into a much sharper curve with little orno distortion of the covering or the underlying stent.

[0059] An additional advantage is that the force necessary to deploy thestent-graft 10 using a coaxial deployment system ought to be less thanfor a fully encapsulated stent. This is due to the reduction in thethickness of covering material. Furthermore, by reducing the amount ofcovering material, the overall profile of the deployment system isreduced, allowing a wider range of applications. Another advantageenjoyed is ease of manufacture compared to stent-graft devices thatplace multiple stent rings over ePTFE tubing. Finally, an advantage overstent-grafts with a single layer of biocompatible material over theentire graft length is that, because a strong bond is created in theencapsulated region, it is possible to transmit a pulling force from oneend of the stent of the present invention to the other via the covering,making it possible to load into a sheath using pulling techniques. Thepreferred bare stent designs (chosen for flexibility and low profile) donot permit transmission of a pulling force in a longitudinal axialdirection. This is because flexibility is increased and profile reducedby removing connections between longitudinally neighbouring struts. Thelimited number of longitudinal connections has inadequate tensilestrength to transmit the pulling force without failure. In the case of atrue single layer covering (without use of adhesive, etc.) pulling onthe covering causes the covering to slip off the stent. In the case ofsutured single layer device pulling on the covering may cause the sutureholes to enlarge and even tear.

[0060] In the case of a biliary stent an especially tortuous deliverypath must be used. There are two main techniques for such delivery. Ifthe stent is delivered transhepatically, it is inserted throughpercutaneous vasculature, through the bulk of the liver and down thehepatic duct where it must make a bend of around 45 degrees between thehepatic and the bile duct. If the stent is delivered endoscopically itenters the bile duct via the papilla and must pass through multiplebends, the most severe of which is about 90 degrees with a 10 mm radius.Clearly, an extremely flexible stent is required. To further illustratethe deployment of the prototypes, FIGS. 4-6 have been provided. FIG. 4shows an overview of the prototypes being deployed into a glass model ofa bile duct using a pistol handgrip delivery system. Note the bend thatthe stent must navigate. FIG. 5 shows a close-up view of a prototype, asit is partially deployed from the sheath. FIG. 6 shows a close-up viewof a fully deployed prototype.

[0061] The “Flexx” stent used in these experiments is a speciallydesigned stent configured for enhanced flexibility. Stents of this typeare cut from tubes of Nitinol shape memory alloy and then expanded on amandrel. The size memory of the device is set on the expanded form. Thedevice is then compressed to the appropriate dimensions of the originaltube for insertion into a patient. Once properly located in the patient,the device is released and can self-expand to the “memorised” expandeddimension. Although the entire device is a single unitary piece as shownin FIG. 9 in its expanded state, this design conceptually comprises aplurality of zigzag ring stents 64 (stenting zones) joined bylongitudinal joining points 62.

[0062]FIG. 10 shows the cut device prior to expansion, to illustratethat each ring stent 64 is attached to each adjacent ring stent 64 (FIG.9) by only a pair of joining points 62. Note the opening regions 60between the joining points 62. It will be apparent that such a structureaffords considerable lateral flexibility to the entire compressedstructure. If there were a larger number of joining points 62, lateralflexibility of the compressed device would be impeded. On the otherhand, the very open structure of the expanded stent (FIG. 9) offerslittle resistance to tissue infiltration.

[0063] These two factors account for the unusual suitability of theFlexx design. The use of a covering of ePTFE or other biocompatiblematerial prevents tissue infiltration despite the very open nature ofthe Flexx design. The use of end encapsulation (as opposed toencapsulation over the entire length of the device) preserves most ofthe inherent flexibility of the design. The use of only a single layerof covering over much of the stent results in a low profile in thecompressed configuration so that the device can be inserted throughsmall bile ducts and other restricted vessels. The use of only a verylimited number of joining points 62 provides the lateral flexibilityrequired for insertion through tortuous bile ducts and other similarlytwisted vessels.

[0064]FIG. 11 is a close-up of a portion of FIG. 9 and shows theadjacent ring stents 64 (stenting zones) and the joining points 62. Eachring stent 64 (stenting zone) is formed from a zigzag pattern of struts54. These struts have the thickness of the Nitinol tube from which thedevice is laser cut with a width, in this embodiment, of about 0.2 mm.There is a joining point 62 between a given ring stent 64 and anadjacent ring stent 64 every third strut 54 with the joining points 62alternating from the left-hand adjacent to the right-hand adjacent ringstent 64 so that six struts 54 separate the joining points 64 betweenany two ring stents 64. Gaps 32 replace the joining points 62 where theintersections of zigzag struts are not joined.

[0065]FIG. 12 shows a close-up of the non-expanded cut structure of FIG.10. Cuts 40, 41, and 42 are regions where the metal has been vaporisedby a computer-controlled cutting laser. The cut 40 between blind cuts 41will expand to form the window 60. Cut 42 forms the intersection point *of the struts 54, which show portions of two ring stents 64. Partiallycut regions 55 define a scrap piece of metal 32′, which is removedfollowing expansion to form the gaps 32. In FIG. 12 the partially shownregion above the cut 40, and above the scrap piece 32′, is a joiningpoint 62. Because a structure with only two joining points 62 betweenadjacent stent rings 64 is too fragile to withstand the tensile stressesendwise on the stent cylinder which are liable to be encountered in theexpansion as from FIG. 12 to FIG. 11, the pieces 32′ act as reinforcingjoining points for the radial expansion process and are not cut out asscrap until afterwards. Following expansion, the scrap pieces 32′ areremoved to form the gaps 32. This structure can be deformed into thereduced diameter flexible structure. It will be apparent that althoughthis structure is described and pictured as having circumferential ringstents 64, the stent zones can also be arranged in a helical manner toachieve the objects of the improved design.

[0066]FIG. 13 shows one end portion of the FIG. 9 stent, again withstruts 54. The end zone E is characterised by a rigidity rather morethan that of the central cylindrical zone of the stent, by virtue of anabsence of cuts and windows 60, as can be perceived in the FIG. 10drawing. At the end vertex 70 of each cell 72 in the firstcircumferential ring of cells of the stent, the material of the stentmatrix is continued into an extending portion 74 (FIG. 14) with a widthcomparable to its thickness dimension so that, in cross-section, it ismore or less square. On each such square section spigot 74 is mounted aspherical Nitinol bead 76 which has a through bore on a diameter of thebead, to receive the spigot 74. The Nitinol bead 76 is welded to thespigot 74. It will be appreciated that, by virtue of the rounded surfaceand greater thickness of the sphere 76 relative to the struts 54, thefree vertices defining the end of the stent, and the end of each cell 72in the end ring of cells of the stent, is less likely to cause trauma inthe bodily tissue in which the end vertices 70 is embedded, than if thespigot 74 and spheres 76 were absent.

[0067] Furthermore, as shown in FIG. 16, the ring of beads 76 bringsadvantages when it comes to loading the stent onto a delivery system,and keeping control of the stent while the stent is being deployed intothe body from the delivery system. Specifically, the ring of relativelythick beads 76 provides a point of purchase for gripping surfaces toimpose forces on the stent, while it is being loaded into a deliverysystem, and while it is being deployed from that delivery system. In oneexample, the beads 76 could be gripped between circumferential surfaces,one inside the stent annulus and one outside the stent annulus, with aspacing between such co-axial surfaces which is wide enough to receivethe thickness of the stent matrix, but does grip the spheres 76 on eachside of the thickness of the stent matrix.

[0068] It has been described above how the form of stent covering allowsthe stent to be subjected to axially directed pulling forces, even whilethe centre section of the stent is extremely flexible. It is to be notedthat, in the present application, the flared end sections of the stentneed not be so flexible, and are not made so flexible, and are thereforebetter adapted to carry axial pulling forces. In the centre section ofthe stent, where the enhanced flexibility renders the stent less able totolerate axial pulling forces, the forces can be shared with the stentcovering. Thus, with the illustrated embodiment, substantial pullingforces can be applied to the ring of beads 76 on one end of the stent,with the stent construction able to transmit such pulling forces all theway to the other end of the stent. It is a significant advantage to beable to maintain full control of the movement of the stent, all from oneextreme end of the stent. Note also that the friction-reducingproperties of PTFE, and the presence of an abluminal PTFE sleeve overmost of the length of the stent, will facilitate loading of the stentinto a delivery system, and deployment of that stent from the deliverysystem, all under the control of a grip on the stent which is appliedonly at one extreme end of the stent length.

[0069] The ring of beads 76 at each end of the stent allows accurateradioscopic tracking of the stent from outside the body.

[0070]FIG. 14 shows in more detail the mounting of a bead 76 on a spigot74 of the stent matrix. The bead 76 has a through bore 80, made by laserdrilling, which has the rectangular cross-section visible in FIG. 15, toaccommodate relatively snugly the rectangular cross-section 82 of thespigot 74. To secure the bead 76 to the spigot 74, laser radiation isused to create a welding bead 82 at the tip of the spigot 74. The stentmatrix, the spigot 74, the bead 76 and the weld bead 82, are all ofnickel titanium alloy.

[0071] Moving now to FIGS. 16 and 17, the stent 12 is shownschematically within the truncated cone of a loading mandrel 90, withits leading end at the narrow end of the cone, tipped by the beads 76.Within the leading end of the stent is a loading rod 92 with a somewhatlarger diameter head 94, the transition from the head 94 to thecylindrical portion 96 of the rod 92 is accomplished by an arcuateshoulder surface 98. The concave outer surface of the shoulder 98 has acurvature which corresponds to the curvature of the beads 76.

[0072] Beyond the narrow end of the truncated cone 90 is a grippingsleeve 100 which has at its gripping end 102 an arcuate grippingshoulder 104, also having a curvature corresponding to that of thespherical surface of the bead 76.

[0073] As can be seen from FIG. 17, drawing the gripping rod 92 down onto the beads 76 achieves an entrapment of the beads 76 in an annulusdefined by the gripping shoulders 98 and 104. With the position of thegripping rod 92 maintained close to the gripping end 102 of the grippingsleeve 100, further pulling down of the gripping rod 92, away from thetruncated cone 90 permits the advancement of the stent 12 into thecylindrical space shown in FIG. 17, within the block 106.

[0074] The block 106 receives a sleeve 108 in which the stent 12 is tobe housed, in a delivery system for placing the stent 12 at a desiredlocation within the body, for location, a catheter. Continued downwardpulling on the gripper rod 92, beyond the position shown in FIG. 17, cancarry the stent 12 fully inside the sleeve 108 of the catheter deliverysystem. Once the stent 12 is within the sleeve, the gripping sleeve 100can be withdrawn forwardly, i.e. downwardly in the FIG. 17 view, whilethe gripper rod 92 can be withdrawn rearwardly from the stent, i.e.upwardly as shown in FIG. 17. Alternatively, once the gripping sleeve100 is withdrawn, it may be possible to withdraw the gripper rod 92 alsoforwardly, given a degree of resilience in the sleeve 108 to allow theenlarged head 94 to slide past the beads 76.

[0075] A variant is shown in FIG. 18. A bead 76 has a through bore 80which receives a spigot 74 defined by two parallel resilient fingers 76a, 76 b formed out of the stent matrix. Each finger has a tip 76 c andeach tip has a re-entrant surface 76 d which abut the outer surface ofthe bead 76 when the tips emerge from the bore 80, to resist reversemovement of the fingers in the bore 80.

EXAMPLE

[0076] An esophageal stent graft was constructed from a Nitinol cylinder0.3 mm thick. A laser controlled by a computer was used to cut amultiplicity of staggered cuts in the cylinder wall, parallel to thecylinder length, to create struts having a width of 0.167 mm. Cutsperpendicular to the length were also made in a mid-length portion ofthe tube length, for selective removal of scrap struts to enhance theflexibility of the mid-length section.

[0077] On a mandrel the tube is brought to its pre-set expandedconfiguration. The end portions of the expanded stent matrix cylinderwere further expanded by the introduction of a tapered annulus betweenthe stent matrix and the cylinder, one at each end of the stent. Thestent matrix, on its mandrel, was then heated in an oven to “set” theconfiguration to be “remembered” by the shape memory alloy. Then, thescrap struts 32′ were removed.

[0078] Following such heat-setting, the matrix was removed from themandrel and a Nitinol bead, with preformed diametral rectangular bore,as shown in FIG. 15, was laser-welded to each spigot (FIG. 14, reference74) present at each end of the stent matrix, to provide 18 beads at eachend of the stent. On a fresh mandrel, the beaded stent matrix was thensubjected to further polishing. On a sintering mandrel, two bands ofePTFE tape were wrapped, at spaced locations corresponding to each endof the cylindrical middle section of the stent matrix. The polishedmatrix was then mounted on the mandrel, overlying the PTFE bands. ThenPTFE tape was wrapped around the stent matrix, to cover the entirecylindrical mid-section of the stent. Then the wrapped matrix, on itsmandrel, was heated in an oven to sinter the PTFE and bond the two PTFEluminal bands to the abluminal PTFE sleeve, through the apertures of thestent matrix. The stent matrix was then ready for loading into adelivery system, as explained in relation to drawing FIGS. 16 and 17.

[0079] For this esophageal stent, beads of diameter 0.95 mm were used.The number of longitudinal slits around the circumference of the stentcylinder was 36. The length of each flared transition section, adjacentto the mid-section of the stent cylinder, was 8 mm. In the expandedconfiguration, the outside diameter of the stent matrix in themid-section of its length was 20 mm. The “crown” ring of beads at eachend had a diameter of 28 mm. The angle of the flared section linking thecylindrical mid-section to the expanded “crown” ends was 15 degrees.Each expanded crown end section had a length of 20 mm. The wallthickness of the Nitinol tube which is the basis of the stent matrix was0.3 mm.

[0080] Those skilled in the art will readily appreciate, from the abovedescription, further advantageous technical effects arising from thetechnical features of the invention described above. While theapplication of the invention to an oesophageal stent graft takesparticular advantage of the technical features described above, they arealso of substantial interest in other applications of stents.

1. An elongate stent matrix which defines a surface in a closed loopsurrounding an elongate flow path, and which is capable of expansionduring deployment in a bodily lumen, from a small diameter deliveryconfiguration to a large diameter lumen wall-supporting configuration;the matrix exhibiting a multiplicity of cells formed from struts, eachof which cells has a length dimension along said flow path, a widthdimension within said closed loop perpendicular to said flow path, and athickness perpendicular to the length and width of the cell, with afirst band of said cells contiguous with a first end of said matrix, anda further band of said cells contiguous with a second end of saidmatrix, opposite the first end; the matrix further exhibiting a firstring which includes at least one extending portion; the matrix beingcharacterized by, an imported bead fixed to said extending portion, saidbead having a thickness greater than that which characterizes the strutsof the matrix, and defining a female receiving portion which receivessaid extending portion as its co-operating male portion.
 2. A matrix asclaimed in claim 1 wherein the extending portion is arranged as ananchor to resist movement of the matrix relative to the tissue of saidbodily lumen.
 3. A matrix as claimed in claim 1 or 2, wherein said firstring defines one end of the matrix, and each extending portion is avertex of one of the first band of matrix cells.
 4. A matrix as claimedin claim 1, 2 or 3, created out of sheet material.
 5. A matrix asclaimed in claim 4 wherein the sheet material is a seamless tube.
 6. Amatrix as claimed in claim 4 wherein the sheet material is initiallyflat, then formed into a tube.
 7. A matrix as claimed in one of thepreceding claims which is a self-expanding stent matrix.
 8. A matrix asclaimed in claim 7 wherein the material of the matrix is a shape memoryalloy.
 9. A matrix as claimed in claim one of claims 1 to 6, which is anon-self-expansible stent, which undergoes plastic deformation onexpansion.
 10. A matrix as claimed in claim 9 wherein the material ofthe matrix is stainless steel.
 11. A matrix as claimed in any of thepreceding claims wherein the material of the beads has anelectrochemical potential which is the same or substantially the same asthat at the matrix material.
 12. A matrix as claimed in claim 11 whereinthe material of the beads and the material of the matrix is the same.13. A matrix as claimed in any one of claims 1 to 10, wherein each beadis electrically separated from the matrix by an electrically-insulatinglayer.
 14. A matrix as claimed in 13 wherein said layer is a polymerlayer.
 15. A matrix as claimed in claim 11 or 12 wherein each bead isfixed to the matrix by welding.
 16. A matrix as claimed in any one ofthe preceding claims wherein each bead is fixed to the matrix by amechanical engagement of co-operating surfaces.
 17. A matrix as claimedin claim 16 wherein the co-operating surfaces on the extending portionare resilient.
 18. A matrix as claimed in claim 17, wherein theresilient co-operating surfaces are fingers lying in the closed loop andelastically deformable to bring the tips of the fingers closer together,to advance into the female receiving portion of the bead.
 19. A matrixas claimed in claim 18 wherein the bead has a receiving portion which isa through bore, and the fingers are long enough to extend through thebore until the tips extend out of the end of the bore.
 20. A matrix asclaimed in claim 19 wherein the finger tips have a re-entrant surface toresist reverse movement of the finger tips into the bore.
 21. A matrixas claimed in any one of the preceding claims wherein each bead is fixedto the matrix by an intervening tie-layer of adhesive.
 22. A matrix asclaimed in any one of the preceding claims wherein each bead isspherical and defines a radially-extending recess to receive the freevertex to which it is fixed.
 23. A matrix as claimed in claim 3, or anyone of claims 4 to 18 as dependent on claim 3, with a said extendingportion at each of spaced intervals around said first ring and with eachbead fixed to a different one of said extending portions.
 24. A matrixas claimed in claim 23 wherein a bead is fixed to every said extendingportion in said first ring.
 25. A matrix as claimed in claim 23 whereina bead is fixed to every other said extending portion in said firstring, proceeding around the circumference of the first ring.
 26. Amatrix as claimed in claim 23, 24, or 25 wherein all the beads in thefirst ring have the same form.
 27. A matrix as claimed in claim 23, 24or 25, wherein the beads are of two forms, one complementary to theother, and arranged around the circumference of the first ring such thatthe two complementary forms alternate, thereby to allow closer approachof the extending portions, to each other in the delivery configuration,than would be the case with an equal bulk of bead material distributedto a set of beads all of spherical form.
 28. A matrix as claimed in anyone of claims 23 to 27, and exhibiting a closed loop of beads at theopposite end of the matrix from said first ring.
 29. A matrix as claimedin claim 28 wherein the arrangement of beads in the first ring and thearrangement of beads in said closed bead loop is the same.
 30. A matrixas claimed in any one of the preceding claims wherein the cells of saidfirst band include flaring, to place the extending portions of the cellsof the first end on a closed loop which has a diameter greater than thatwhich defines the end zone of the matrix in which the first end islocated.
 31. A stent comprising a stent matrix as claimed in any one ofthe preceding claims.
 32. A stent as claimed in claim 31 which includesa covering of at least a portion of the matrix.
 33. A stent as claimedin claim 32 wherein said portion includes a mid-length portion of thematrix and excludes end portions of the matrix.
 34. A stent as claimedin any one of claims 31 to 33 wherein the matrix has a diameter betweenits ends which, at its large diameter configuration, is smaller than thediameter of at least one end portion of the matrix.
 35. A stent asclaimed in claim 34, having in its large diameter configuration threedistinct diameter zones, namely, (1) a mid-length more or less constantdiameter, (2) an end zone at each end of the matrix, each end zonehaving a more or less constant diameter greater than said mid-lengthzone; and (3) a transition zone of changing diameter, steplessly joiningthe mid-length zone to each end zone.
 36. A stent graft based on thestent as claimed in any one of claims 31 to
 35. 37. A stent as claimedin any one of claims 31 to 35, or a stent graft as claimed in claim 36,for placement in the human esophagus.